Allies of the U.S. auto industry stepped up a campaign yesterday to soften strict vehicle fuel-efficiency mandates in proposed energy legislation before the Senate, even as momentum for the tougher measures continued to build.

Sens. Carl M. Levin and Debbie Stabenow, the two Democrats from Michigan, and Christopher S. Bond (R-Mo.) are leading the effort to craft an amendment that opponents say would water down measures already approved by the Senate Commerce Committee.

The Senate bill's current provisions would require automakers to meet combined fuel-efficiency targets for cars and trucks of 35 miles per gallon on average by 2020, with 4 percent annual increases from 2021 to 2030. Levin wants 36 mpg on average for cars and 30 mpg for trucks by 2022, without further increases. The final text of Levin's proposal has not been released.

Midwestern lawmakers are under pressure back home -- especially in Michigan, where downsizing by General Motors, Ford and Chrysler has contributed to a three-year-old, one-state recession.

At a news conference yesterday, Levin and Stabenow expressed frustration that some senators overlook what effect the fuel-economy mandate in the proposed bill could have on the auto industry.

"Michigan has been hurt very badly," Levin said, "The bias that exists here against the American automobile industry is a hurdle."

Stabenow complained about a meeting this week in which senators were simply "throwing out" fuel-efficiency figures, some as high as 40 mpg. "These numbers at some point become arbitrary," she said. "There's not a context based on what's achievable."

A vote on the Levin-Bond amendment is expected next week. Sen. Byron L. Dorgan (D-N.D.), who is opposing the auto industry, said this week that he thought there were enough votes to defeat the attempt to weaken the legislation.

Stabenow said the vote could be close. "We don't pretend we have it in the bag," she said. "We know it's going to be tough."

To win support, Levin yesterday said he dropped a provision that opponents had "mischaracterized" as a loophole that would allow auto companies to circumvent new rules. A Senate staff member, who spoke on the condition of anonymity, said leaders of the push for an amendment were making changes to make it more palatable to Senate colleagues.

Auto lobbyists said they were encountering stiff resistance on Capitol Hill. They said they felt like the industry was being punished for what one called the "sins of the past" -- successfully beating back attempts to make major changes to the nation's vehicle mileage laws.

One lobbyist said lawmakers and their aides expect automakers to deliver new vehicles that are the same size as today's but consume less fuel.

Yesterday, Sen. Dianne Feinstein (D-Calif.) defended the current bill, arguing that it would provide flexibility for automakers. "There are all kinds of dire warnings," Feinstein said. "The fact of matter is that Detroit has done nothing about mileage efficiency for the past 20 years, and the time has come."